There is a great deal of interest in developing economical means for the synthesis of useful compounds by the chemical modification of relatively simple organic molecules, such as olefins (alkenes). Organic compounds that lack symmetry have two different configurational forms, or enantiomers. Frequently, a specific configuration or enantiomer may be preferred for a particular application. Chemical modifications can be conducted more efficiently when a catalyst is included. The interest in obtaining specific configurations of organic molecules by efficient means has made the development of catalysts that mediate enantioselective group transfer to relatively unfunctionalized olefins an important goal in organic chemistry.
Catalytic systems are known to the art, but these systems are unsatisfactory because the substrates must have specific functional groups to achieve the pre-coordination required for high enantioselectivity. This restriction is lifted when the stereoselectivity relies solely on non-bonded interactions. Extensive research in the enantioselective atom or group transfer to unfunctionalized olefins has led to the recent discovery of synthetically useful catalysts for a variety of chemical modifications, including for example the addition of hydroxyl groups, epoxy groups, and alkyl groups.
Ligated systems with C.sub.2 -symmetry have been found to be useful for asymmetric reactions and other catalytic transformations. The [14] aneN.sub.4 macrocycles (cyclams) and salen based ligands have been synthesized and found to stabilize unusual geometries and oxidation states of transition or main group metals. These systems are effective in catalyzing epoxidation reactions; however, the synthesis of such ligands is generally a laborious process. This has led to an interest in the development of chiral ligands that can be easily synthesized from inexpensive chiral starting materials. In addition, there is interest in developing an epoxidation method that uses an inexpensive and environmentally safe oxygen atom source.
Chemists and enzymologists have been-studying and attempting to duplicate the high efficiency of metalloproteins for over 70 years. Of particular interest are the metalloporphyrins, which display a wide variety of chemical reactivity, principally due to their ability to complex almost any of the transition or main group metals (Porphyrins and Metalloporphyrins, Smith and Elsevier, Amsterdam, 1975; The Porphyrins, Dolphin, Academic Press, New York, 1979). Most of the metalloporphyrin chemistry has been developed in a "self-governing field" far removed from the practical constraints imposed by the requirements of catalysis and organic synthesis (Menuier, B., Bull Soc. Chim. Fr. 578-594, 1986). Exquisitely powerful and selective catalysts have been designed based on the porphyrin ring system (Holm, R. H., Chem. Rev. 87: 1401-1449, 1987), but the lengthy synthesis required to produce these metal-binding moieties may preclude their widespread use as practical chemical catalysts. Another approach that has been employed in the development of catalysts is to use a peptide backbone (Margerum, D. W., Pure Appl. Chem. 55: 23-34, 1983), or a peptide-derived macrocycle (Hsiao and Hegedus, J. Org. Chem. 62: 3586-91, 1997; Busch, D. H., Acc. Chem, Res. 11: 392-400, 1978) to bind metals.
What is needed in the art is a ligand system that can be readily synthesized and which is capable of catalyzing the enantioselective transfer of an atom or group to an unfunctionalized olefin.